There are hundreds of different types of oscilloscope probes. If used properly, oscilloscope probes enable the engineer or technician to connect signals on the circuit under test to the oscilloscope so that signals can be captured and measured easily and accurately.
Of the many types of oscilloscope probes, they can all be broken down into a few main categories. These categories are current probes, voltage probes, active probes, passive probes, high-voltage probes, and probes just for making DC measurements.
Basic knowledge of the probe types and their performance capabilities is important because their operation can affect the whole system under test and measurement results obtained.
Functions of an Oscilloscope
The main purpose of an oscilloscope (scope) is to measure amplitude over time. It’s the amplitude that can change depending on what type of probe is used. Where a voltage probe is often used to measure voltage over time, and a current probe is used to measure current over time.
Additionally, math functions in the scope can be used to multiply voltage and current to look at power over time. You can integrate current over time and look at charge over time. There is an endless variety of measurements that can be made on electronic signals when using the right scope and probe combination.
Scopes normally have a 1MΩ input impedance, and often, there’s a 50Ω capability as well. Passive probes typically use a 1 MΩ impedance while a few are designed to operate using 50Ω.
Main Types of Scope Probes
The main types of scope probes are:
- Voltage or Current
- Passive or Active
- Single-ended or Differential
- Active probe
Most Common Type of Scope Probe
The most common type of scope probe is the passive voltage probe. There are two varieties of the passive voltage probe:
- Times one – identified as x1 or 1x
- Times 10 – identified as x10 or 10x.
Some passive voltage probes are selectable between 1x and 10x with a simple flip of a switch located on the probe’s body.
Each type has its benefits and limitations.
Note: There are some 100x probes, but these are not as common as the 1x and 10x probes.
The 1x probe has better sensitivity with its 1MΩ input impedance whereas the 10x probe provides a reduction in input signal by 10, with its characteristic 10MΩ of impedance.
These impedances are correct at DC and very low frequencies but reduce significantly with increasing frequency. These limitations are discussed shortly.
Passive Probe Limitations
Passive probes present an impedance to the circuit under test. For a 1x passive probe, the impedance presented to the circuit under test is 1MΩ at DC, and for a 10x passive probe, it’s around 10MΩ total.
However, there is some capacitance involved with passive probing, and it’s this capacitance that can have a detrimental loading effect on the circuit under test. Even at low frequencies, capacitive loading can severely skew measurement results obtained.
To compensate for this capacitance loading effect, a capacitor is often placed internally across the largest portion of resistance in the 10x passive probe, allowing for adjustment. It’s essentially a circuit with a capacitive potential divider adjusted to provide a flat frequency response. This adjustment can be made at the tip of the probe or where the probe connects to the scope. This adjustment can be seen on the probe body.
Pro Tip: Use the scope’s internal square wave generator to provide the correct “flattened” signal to the probe. If the scope shows a non-square signal, then probe adjustment is necessary until the signal is squared up, i.e., with no rounded edges or overshoot is observed on the scope.
Where and how the probe is grounded to the circuit under test is also extremely important. It’s best to keep the ground as short as possible to minimize loop area. The long lead grounds with alligator and other similar types of “big” clips can result in unwanted ringing and noise in the measured signal. Eliminating this big loop removes signal ringing and unwanted noise. Use of grounds springs or other similar zero lead length grounding devices are a must if probing any high frequencies signals.
Bandwidth of the Probe
The bandwidth of the probe is also an important part of the complete measurement system. It is recommended that the probe’s bandwidth is at least 1.5 times the maximum of the scope’s bandwidth.
Current probes are a very useful type of scope probe. They are used to obtain a measurement indirectly, without loading down or otherwise adversely affecting circuit operating. The probe body is simply clamped around the cable that is carrying the current, and the result current is displayed on the scope display.
When there are kilo-volts involved, the passive type of 1x and 10x voltage probes are no longer suitable, and a high-voltage probe is required. This type of probe steps the voltage down to a safe level so that the scope can measure it. Often, high-voltage probes have special insulated ends, and they may be specified with divider ratios of x50, x100, or x1000.
These probes can be either a voltage or a current type of probe, with the voltage type being the more common of the two types. These types of probes have an active amplifier in their tips. This enables this type of probe to have a very high resistance and low capacitance for low levels of circuit loading. These types of probes are very useful for working on very high frequency signals and circuits, typically around 500 MHz or more, where the run-of-the-mill passive types of probe performance becomes an issue.
Other Pro Tips
- Make sure you are using a probe with high enough voltage rating for the circuit you’re probing.
- Don’t forget to compensate your probes to the actual scope signal source.
- Make sure the system bandwidth is sufficient by using a probe that has a bandwidth of 1.5x the scope’s bandwidth.